1. Field of the Invention
The present invention relates to the fabrication of integrated circuits, and more particularly to an apparatus and process that facilitates the uniform overflow of a liquid containing contaminants from a process tank and prevents the contaminated liquid from reentering the process.
2. Description of the Prior Art
Integrated circuit devices are produced on semiconductor wafers where each wafer can produce a multitude of integrated circuit devices. The exact number of devices that can be produced on any single wafer depends both on the size of the wafer and the size of the devices being produced thereon. In the production of integrated circuit devices, the importance of minimizing the amount of contaminants on the wafers at all stages of processing has long been recognized in the industry. Contaminants cause otherwise properly processed devices to not function properly upon completion of the production process. As a result of this contamination, the yield of properly functioning devices obtained from any given wafer decreases.
Moreover, cleanliness requirements have become increasingly important as a result of the devices becoming more and more miniaturized. When dealing with reduced size devices, the ratio of the size of a contaminant compared to the size of a device is greater, resulting in an increased likelihood that a contaminated device will not function properly. Thus, increasingly stringent cleanliness requirements are needed. As a result, improved semiconductor wafer processing techniques that reduce the amount and size of the contaminants present during wafer production are highly desired.
One method used to clean wafers that is known in the art is overflow. In overflow washers, wafers are cleaned by supplying a rinsing liquid through the bottom of a tank in which the wafers are located. The liquid is continuously supplied so that the liquid eventually fills the tank and overflows its sides. The theory behind the cleaning function of overflow washers is that as the tank fills with fresh, clean liquid, the dirtier liquid that contains contaminants that have been removed from the wafers is removed from the upper portion of the tank by overflow, the dirty liquid being continuously replaced with fresh, clean liquid. Additionally, many overflow washers also include a bubbler which introduces a stream of nitrogen bubbles into the bottom of the tank to enhance the rinsing action of the flowing liquid and which helps carry contaminants to the surface of the liquid where they will be removed from the tank by the overflowing liquid. Another technique utilized in the art to increase the cleanliness of the liquid used in overflow washers is to facilitate uniform overflow of the liquid from the tank. Uniform overflow is accomplished in the art by providing a multitude of peaks at or near the top of the tank walls, these peaks in turn forming a corresponding recess between each pair of peaks. The surface liquid overflows the tank walls through such recesses, causing the liquid to overflow the tank walls from all sides and thus facilitating a uniform overflow of surface liquid from the tank. Because a uniform overflow of liquid more effectively removes contaminants located on or near the surface of the liquid than a non-uniform overflow of liquid, the liquid remaining in the tank contains less contaminant and is cleaner.
While employing recesses does facilitate uniform overflow, the surface liquid overflows the tank walls only through the recesses. This results in a small area of surface fluid maintaining a steady state (i.e. a zero flow rate) at each peak of the tank walls. This lack of flow near the peaks of the tank walls results in contaminants that are present in the surface tension of the liquid adhering to the peaks of the tank walls. As the surface liquid continues to overflow the tank through the recesses, a significant amount of contaminants can build up at the peaks. When the supply of liquid to the tank is stopped and the remaining liquid is drained from the tank, some of these contaminants will migrate back over the surface of the liquid. As the surface level of the liquid lowers past the wafers, some of these contaminants often come back into contact with the cleaned wafers, resulting in re-contamination of the wafers and an increase in the number of devices that will not function properly.
Thus, there is a need for a system that can facilitate the uniform overflow of liquid from a tank without allowing contaminants to reenter that area of the tank in which the wafers are located.
These needs and others are met by the present invention which comprises in one aspect system that facilitates the uniform overflow of liquid from a process tank while preventing contaminants from reentering the process tank upon draining the process tank. The system comprises an inner weir having a top surface; an overflow wall having a top with at least one recess, the at least one recess having a bottom; wherein the top surface of the inner weir is below the bottom of the at least one recess; and a structure connecting the overflow wall and the inner weir so as to form a drainage basin, the drainage basin having at least one drain hole.
Preferably, the top surface of the inner weir has an inside edge and an outside edge, the top surface of the inner weir being tapered downwardly from the inside edge to the outside edge, wherein the inside edge of the top surface is below the bottom of the at least one recess. The at least one recess can be saw-toothed, rectangular, or semi-circular.
Optionally, the system also comprises a drain valve that is fluidly connected to the at least one drain hole. The drain valve has an open and closed position so that the at least one drain hole is hermetically sealed when the drain valve is closed and allows fluid to freely flow through the at least one drain hole when the drain valve is opened.
In the preferred embodiment, the system of invention comprises an inner weir having a top surface; an overflow wall having a top with at least one recess, the at least one recess having a bottom; wherein the top surface of the inner weir is below the bottom of the at least one recess; and a structure connecting the overflow wall and the inner weir so as to form a drainage basin, the drainage basin having at least one drain hole; a drain valve fluidly connected to the at least one drain hole, the drain valve having an open and closed position so that the at least one drain hole is hermetically sealed when the drain valve is closed and allows fluid to freely flow through the at least one drain hole when the drain valve is opened; wherein the top surface of the inner weir has as inside edge and an outside edge, the top surface of the inner weir being tapered downwardly from the inside edge to the outside edge, wherein the inside edge of the top surface is below the bottom of the at least one recess; and wherein the at least one recess is saw-toothed.
In another embodiment, the invention is a process tank comprising the system of invention described above. The process tank can be a rinsing tank, a drying tank, or a chemical treatment tank.
In even another embodiment, the invention is a method of facilitating uniform overflow of liquid from a process tank while preventing contaminants from reentering the process tank upon draining the process tank. The method comprises the steps of: providing a process tank comprising the system of invention described above; supplying a liquid to the process tank wherein the liquid comprising contaminants overflows the inner weir, fills the drainage basin, and overflows the overflow wall through the at least one recess of the overflow wall; and wherein upon discontinuing the supply of liquid to the process tank, the contaminants do not reenter the process tank.
Optionally, the at least one drain hole used in this method is fluidly connected to a drain valve having an open and closed position so that the at least one drain hole is hermetically sealed when the drain valve is closed and allows fluid to freely flow through the at least one drain hole when the drain valve is opened.
Preferably, in performing the method above, the drain valve is opened essentially in concurrence with discontinuing the supply of liquid to the process tank.
The top surface of the inner weir can have an inside edge and an outside edge, wherein the top surface of the inner weir is tapered downwardly from the inside edge to the outside edge, wherein the inside edge of the top surface is below the bottom of the at least one recess. Also, the at least one recess can be saw-toothed, rectangular, or semi-circular.
In the preferred embodiment, the method of invention comprises providing a process tank comprising the system of invention described above; supplying a liquid to the process tank wherein the liquid comprising contaminants overflows the inner weir, fills the drainage basin, and overflows the overflow wall through the at least one recess of the overflow wall; and wherein upon discontinuing the supply of liquid to the process tank, the contaminants do not reenter the process tank; wherein the at least one drain hole is fluidly connected to a drain valve having an open and closed position so that the at least one drain hole is hermetically sealed when the drain valve is closed and allows fluid to freely flow through the at least one drain hole when the drain valve is opened; wherein the top surface of the inner weir has an inside edge and an outside edge, the top surface of the inner weir being tapered downwardly from the inside edge to the outside edge, wherein the inside edge of the top surface is below the bottom of the at least one recess; and wherein the at least one recess is saw toothed.